Lecture 12

Question 1

requirements for useable water

Answer 1

• no chemical contamination
• no microbial contamination
• Specific pH range
• Removal of e.g. Cu, Mo, Ag, Cd, Sn, P for PEM electrolysis

Question 2

Paragons Water Purification System

Answer 2

• called ICICLE
• captures & coarsely purifies lunar water through freeze distillation
• Ionomer-membrane Water Processing technology -> broadband contaminant filtration
• IWP bundles contain more than 1000 Nafion tubes that selectively transport water vapour

Question 3

H2O electrolysis

Answer 3

• overall reaction: 2H2O(l)->2O2(g)+O2(g)
• minimum potential difference 1.23 V
• soluble electrolyte needed for proper ion flow
• H3O+ formation at Anode & Oh- formation at cathode (get neutralised)

Question 4

Types of H2O electrolysis

Answer 4

• Alkaline Electrolysis (AEL)
• Proton Exchange Membrane (PEM) Electrolysis
• Solid Oxide Electrolysis (SOE)

Question 5

Alkaline Electrolysis (AEL)

Answer 5

• membrane ensures separation of reaction products
• Hydroxide ions are charge carrier
• 70-80% efficiency
• Common membranes
  
  • cannot be used under pressure
  • only tolerate low current densities
• Classical setup has separator & two separate electrodes
• H2O separation at cathode
• Russian system was used on module Zvezda on ISS

Question 6

AEL - Zero-gap setup

Answer 6

• doesn’t have electrodes as own part & a thicker CL directly at the membrane
• typically results in larger current densities & higher cell efficiencies -> liquid electrolyte resistance -> no addition in ohmic losses
• requires porous electrodes & transport layer structures to maintain gas phase separation

Question 7

AEL - dual-membrane cell

Answer 7

• could potentially be suitable for low-g applications
• aims to improve gas phase separation as compared to zero-gap setup
• electrochemically accessible surface area of electrode needs increasing, usually achieved by utilising foams, fibres, meshes or felts as electrodes

Question 8

Proton Exchange Membrane (PEM) Electrolysis

Answer 8

• higher current density & pressure than AEL
• Operates at lower T than AEL
• H2O separation at anode
• Protons are charge carrier
• acidic working environment -> corrosion-resistant iridium or platinum electrodes
• gas-proof, proton-conducting plastic membrane is used to separate the gas products

Question 9

Cell voltage

Answer 9

• sum of:
  
  • reversible potential
  • activation over potential at anode & cathode
  • ohmic overpotential
  • diffusion over potential
  • bubble overpotential (-> in microgravity buoyancy isn’t strong enough to detach bubbles); main problem

Question 10

Solid Oxide Electrolysis (SOE)

Answer 10

• is a Solid Oxide Fuel Cell running in regenerative mode
• H2O separation at cathode
• T up to 1000°C using superheated water vapour
• Efficiencies about 80% possible
• Uses porous cathode -> steam can react with H2 & O2 ions
• problems include anode delamination due to O2 pressure build-up

Question 11

Countermessures to bubble detachment problem

Answer 11

• Nanopatterning
  - limited applicability to active electrodes with functionalisation & passivation)
• UV irradiation
  - proven succesfull for photoelectrolytical cells)
• Pressure swings
• Vibration

Question 12

CO2 Electrolysis - MOXIE

Answer 12

• SOE process with CO2
• compact
• produces oxygen
• CO2 is split at the cathode
• operational from 2021 to 2023
• typically generated 6-8 g/h of Oxygen (highest was 12g/h)

Question 13

Risks of MOXIE process

Answer 13

• electrode coking due to carbon instead of carbon monoxide production
  -> Voltage & gas supply that prevent C formation out of CO need to be maintained

Question 14

Biomining

Answer 14

• also called bioleaching/biohydrometallurgy
• Goal: extract metals
• successfully employed on Earth
• Mainly useful for low-grade ores
• can use Bacteria, Fungi, Biorock
• Microorganisms are grown on the regolith, forming a leachate with dissolved metal ions
• Challenges
  - Survival of bacteria in space
  - Requires water

Question 15

Bacteria

Answer 15

• Acidithiobacillus ferrooxidans use elementary sulfur, tetrathionate & ferrous iron as electron donors
• able to grow anaerobically & solubilise metals from lunar & Martian regolith simulants
• Leaching yield depends on mineralogy & grain sizes

Question 16

Fungi

Answer 16

• Penicillium simplicissimum used
• can extract Aluminium, Iron, Calcium, Mg, Ti, Manganese, Sodium, Potassium, Phosphorus